For certain drugs whose molecules contain chiral, i.e. asymmetric carbon atoms it has been demonstrated that the therapeutic effect is tied to a certain enantiomer, i.e. an optical isomer. In such cases where the clinically used drug comprises either a racemate, i.e. equal parts of two enantiomers, or other mixtures of enantiomers of which only one of the enantiomers carries the clinically desired effect, the "inactive" form can contribute to side-effects without any corresponding therapeutic effect contributions. The use of only the "active" enantiomer results in therapeutical improvements for such drugs. In the development phase as well as in the production phase of drugs of the above character a technique is required which enables the separation of enantiomers in an analytical as well as in a preparative scale. In that the development of drugs goes towards a more selective therapy with the purpose to minimize the side-effects, a large part of the drugs registered today are strictly sterically defined, i.e. they contain only enantiomers with the desired pharmacological effect. This circumstance causes a strongly increasing need for separation methods for checking synthesis processes in the production of such drug substances and drug preparations. There is also an increasing need for methods that would enable determinations of the contents of enantiomers in body fluids such as plasma and urine. Such determinations can be used for pharmacokinetic characterization of enantiomers to evaluate whether therapeutical advantages can be obtained if only one enantiomer is administered.
Presently, the possibilities are limited for direct separation of enantiomers of racemic drug substances by means of liquid chromatography. The majority of the described methods deal with the resolution of racemic amino acids. Three principally different approaches are known for separating enantiomers:
The oldest method is an indirect method and is based on the reaction of a racemic substance with a pure enantiomer of another substance, wherein so called diastereomeric derivatives are formed. The diasteromers can then be separated on a non-chiral column. This technique is however time-consuming and can give rise to serious errors upon the determination of the optical purity of e.g. a drug substance due to racemizing of the used chiral reagent or due to the fact that the enantiomers react with different rates with the chiral reagent.
Chiral stationary phases have also been used for the separation of enantiomers. Such phases have been produced by immobilization of a chiral compound on to a solid phase. However, the majority of these phases exhibits an important limitation as concerns the usability for different types of chemical substances. For many of the phases it is, moreover, supposed that test substances are present in some form of derivatives in order to be able to use the method. This means that the preparative use of such methods are limited.
Enantiomers of preferentially amino acids have also been separated by addition of a chiral complexing agent to the mobile phase in a liquid chromatographic separation system. However, the addition of the complexing agent to the mobile phase makes the method unsuitable to use for preparative purposes. Moreover, the method is unfavourable from an economical point of view in view of the large amounts of the chiral complexing agent that are used.